1. Field of the Invention
The present invention relates to communication between a plurality of communication terminals and a server, and more particularly to a communication system in which a plurality of communication terminals sequentially transfer data to a server during a transmission permissible period assigned to each of the communication terminals, the communication terminal, the server, and the data transfer control program.
2. Description of the Related Art
Recently, a demand for speeding up a network at a lower cost is increasing with prevalence of a multi-media communication service including sound, image, and Internet. The demand is increasing not only in a basic network but also in a subscriber access network, and application of the Ethernet (R) technique which has been developed as the LAN technique, to a public network, especially to a subscriber access network, is advanced.
FIG. 38 is a view showing the structure of the E-SS (Ethernet(R)-Single Star) system by way of example in the case of applying the Ethernet (R) technique to a subscriber access network.
An E-SS system 100 of FIG. 38 comprises optical network units (communication terminals) 110 to 112 respectively connected through fibers (optical fibers) 140 to 142 and optical line termination (server) 120. Client devices 150 to 152 are respectively connected to the optical network units 110 to 112, and a local switch 160 is connected to the optical line termination 120, which is connected to a metro network through the local switch 160. A point-to-point connection is established between the respective optical network units 110 to 112 and the optical line termination 120 through the respective fibers 140 to 142.
Each of the optical network units 110 to 112 comprises a MAC processing unit 113, an 8B10B coding unit 115, a signal serializing unit 116, and a physical interface 117. The optical line termination 120 comprises a MAC processing unit 123 and physical layer processing units 130 to 132 each consisting of an 8B10B decoding unit 125, a signal paralleling unit 126, and a physical interface 127.
For the sake of easy description, there show only the respective function units concerned about the upstream data transfer from the optical network units 110 to 112 to the optical line termination 120.
There are a lot of cases of using the gigabit Ethernet(R) of the full-duplex mode for the fibers 140 to 142, according to the demand for speeding up and the restriction of the arrival distance.
In the E-SS system 100 in the case of following the definition of IEEE 802.3, a frame is transferred as follows from the optical network units 110 to 112 to the optical line termination 120.
Upon receipt of MAC frames from the client devices 150 to 152, the MAC processing unit 113 processes their addresses and transfers the frames to the 8B10B coding unit 115.
The 8B10B coding unit 115 encodes the received MAC frames in order to restrain the deterioration of the signals on a transmission channel. More specifically, as for the MAC frame, the data for every 8 bits is converted into each code of 10 bits including 1 and 0 half and half. The data string converted into 10 bit-code is transferred to the signal serializing unit 116.
The signal serializing unit 116 converts the code string for every 10 bit into a serial signal and transfers it to the physical interface 117. The physical interface 117 transfers the received serial signal string to the fiber 140.
The serial signal string is transferred to the physical interface 127 of the physical layer processing unit 130 of the optical line termination 120, through the fiber 140. The physical interface 127 transfers the received signal string to the signal paralleling unit 126. The signal paralleling unit 126 converts the serial signal string into parallel code strings for every 10 bits, and transfers the same to the 8B10B decoding unit 125. The 8B10B decoding unit 125 decodes the code strings for every 10 bits to the data for every 8 bits.
The decoded MAC frames are transferred to the MAC processing unit 123. The MAC processing unit 123 processes the addresses of the received MAC frames and transfers the same to the local switch 160.
In this way, in the E-SS system 100 having a point-to-point connection between the respective optical network units 110 to 112 and the optical line termination 120, upon receipt of the MAC frames from the respective client devices 150 to 152, the respective optical network units 110 to 112 transfer the received frames to the optical line termination 120 sequentially, thereby transfer the frames in an upstream direction.
As a subscriber access network capable of reducing the cost more than the E-SS system 100 of FIG. 38, the EPON (Ethernet(R)-PON) system of the point-to-multipoint PON structure receives much attention.
FIG. 39 is an outline of the conventional EPON system 200. In the EPON system 200 of FIG. 39, each optical network unit 210 to 212 is designed to have a MAC control unit 114 in addition to the structure of each optical network unit 110 to 112. Further, the optical line termination 220 is designed to have a MAC control unit 124 in addition to the structure of the optical line termination 120 of FIG. 38, and the physical layer processing units 130 to 132 corresponding to the respective optical network units in FIG. 38 are integrated into one physical layer processing unit 130.
The EPON system 200 is different from the E-SS system 100 in that a point-to-multipoint connection is established between the respective optical network units 210 to 212 and the optical line termination 220. Namely, by providing a passive signal combining/separating unit 230 on a communication channel, the respective optical network units 210 to 212 are connected to the passive signal combining/separating unit 230 through the respective fibers (optical fibers) 140 to 142 in a multipoint way, and the passive signal combining/separating unit 230 is connected to the optical line termination 220 through a shared fiber 240.
Constituted as mentioned above, the EPON system 200 can share the physical layer processing unit 130 of the optical line termination 220 among the several optical network units 210 to 212, thereby saving the cost.
In the EPON system 200, in order to avoid a signal collision in the passive signal combining/separating unit 230 within the shared fiber 240 of the optical line termination 220 shared by the optical network units 210 to 212, the MAC control unit 124 of the optical line termination 220 makes the respective optical network units 210 to 212 execute the data transfer to the optical line termination 220 within the respective frame transmission permissible periods previously assigned, thereby controlling the upstream frame transmission.
The above-mentioned EPON system 200, however, has the following problems as for the frame transfer, although it has the physical structure capable of saving the cost.
If the conventional EPON system 200 follows the IEEE 802.3, it is necessary for the respective optical network units (communication terminals) 210 to 212 to transmit a signal during a period other than the frame transmission permissible period previously assigned. Therefore, in the passive signal combining/separating unit 230, signals from the respective optical network units 210 to 212 always collide with each other and the optical line termination (server) 220 cannot receive the correct signals.
FIG. 40 and FIG. 41 are signal strings to be transferred by the EPON system 200. The signal string 300 shown in FIG. 40 is a signal string to be transferred from the MAC processing unit 113 to the 8B10B coding unit 115.
The signal string 300 consists of MAC frames 330 to 332, preambles 340 to 342 respectively added to the MAC frames 330 to 332, and inter-packet gaps (Inter Packet Gap: IPG) 350 and 351 interposed between the respective MAC frames 330 to 332, and the signal string 300 is transferred during the frame transmission permissible period.
The signal string 300 is converted into the signal string 310 by the 8B10B coding unit 115. In the signal string 310, the portion from the preamble 342 to the MAC frame 330 of the signal string 300 becomes the 8B10B coding signal 370, and a START signal 360 is added to the signal. Idle signals 371 and 372 are transferred to the non-transmission portion of the signal string 300.
In these ways, since signals (idle signals 371 and 372) are transmitted from the respective optical network units 210 to 212 also during the period other than the frame transmission permissible period, the signals from the respective optical network units 210 to 212 collide in the passive signal combining/separating unit 230, and therefore, the optical line termination 220 cannot receive correct signals.
Further, in the conventional EPON system, as mentioned above, since the signals collide in the passive signal combining/separating unit, correct signals cannot be received in the optical line termination, and further, synchronization cannot be taken for a short period in the physical interface 127 of the optical line termination 220 because the preamble (EPON preamble for synchronization) is coded by the 8B10B coding unit 115 in the EPON system 200, thereby deteriorating the transfer efficiency.
Namely, the signal string 300 is 8B10B-encoded and converted into the signal string 310 shown in FIG. 41. In the signal string 310, a bit pattern [1010101010 . . . ] no longer exists, by 8B10B-encoding the preambles 340 to 342 of the signal string 300. Therefore, synchronization cannot be taken for a short period in the physical interface 127 of the optical line termination 220.